Magnetic recording media generally include a binder dispersion layer comprising a binder and a pigment overlying a substrate, wherein the pigment is dispersed within the binder. Typically, the pigment is a magnetizable pigment comprising small, magnetizable particles. In some instances, the medium may be in the form of a composite having both back-coat and front-coat binder dispersion layers, although the pigment in the back-coat may or may not be a magnetizable pigment.
It has become desirable to have as high a loading of magnetizable pigment in the magnetic recording media as is reasonably possible. It is often preferred to have a binder dispersion comprising from about 70% to 85% by weight magnetizable pigment relative to the binder with as many magnetizable particles per unit area or unit volume as possible. It is also preferred to have a binder dispersion in which the magnetizable pigment comprises a plurality of small particles having a relatively high specific surface area. Higher pigment loading has the potential to provide high density magnetic recording media capable of storing more information.
Problems, however, remain in the art concerning magnetic recording media having a relatively high loading of magnetizable pigment. To begin with, magnetizable pigments tend to agglomerate, and they are difficult to properly and fully disperse within the binder. Wetting agents, or dispersants, are often employed to facilitate such dispersion. For higher pigment loading, i.e., the use of greater amounts by weight and number of magnetizable particles, greater amounts of such dispersants are required, which is not always desirable. There are a number of reasons for using as little dispersant as possible. Dispersants tend to soften binder systems and decrease their toughness They may cure poorly. In addition, dispersants are hydrophilic and tend to increase binder water absorbance. Additionally, binder dispersions can be more readily and reproducibly prepared when no dispersant is used. Further, excess dispersant may have a tendency to bloom from a cured binder dispersion over time, leading to contamination of a recording head or the like, or causing a change in the physical or chemical characteristics of the media.
To help alleviate these problems with high pigment loading, binder compositions having internal dispersants have been developed. Such compositions comprise polymers with functional moieties pendant from the polymer backbone that help disperse the magnetizable pigment. As a result of using these compositions, less dispersant or in some instances no dispersant is needed for dispersion of magnetizable pigment in the binder.
A large amount of attention has been focused by persons skilled in the magnetic media art on polyurethanes and other polymers containing sulfonate, carboxylate, phosphate, and ammonium functionality in order to produce pigment dispersing polymers. For example, U.S. Pat. No. 4,152,485 discloses sulfonated polyurethanes. Some of the polymers described in the art can disperse iron oxide and other pigments. The dispersing groups are without exception highly polar, i.e. either salts or strongly acidic or basic.
Polymers containing highly polar dispersing groups can be disadvantageous when high humidity conditions are encountered since it has been demonstrated that the binder hydrolysis rate increases and it is speculated that coatings formed therefrom can soften and swell and friction can increase under high humidity conditions.
Copending U.S. application Ser. No. 07/635,858 (assigned to the assignee of the present case) incorporated by reference herein, discloses a heterocyclic thione functional polyurethane polymer and its use in magnetic recording media. The heterocyclic thione functional polymer is hydrophobic and thus capable of withstanding high humidity conditions without experiencing undesirable water absorption in addition to having good pigment dispersing ability, curability, and good mechanical properties (i.e., toughness, abrasion resistance, etc.).
Blends of some polyurethanes with some vinyl chloride resins are known in the magnetic binder patent art. A need exists for a blend which is capable of producing high quality pigment dispersions without need for low molecular weight dispersants. Vinyl chloride resins containing polar functionality, especially sulfonate or ammonium salt, are capable of producing good dispersions but, for most product applications, these resins lack the desired toughness and flexibility. Polyurethanes may be blended with these resins in order to improve their properties but care must be taken to preserve dispersion quality. For example sulfonated polyurethanes can produce good dispersions when blended with sulfonated vinyl resins, but are incompatible with ammonium vinyl chloride resins due to ion exchange. Carboxylated polyurethanes are compatible with ammonium vinyl chloride resins but the commercially available carboxylated polyurethanes are known to produce relatively poor dispersions by themselves and in general they degrade the dispersing capability of ammonium vinyl chloride resins upon blending. A need exists for polyurethane vinyl chloride polymer blends where the action of the polyurethane enhances and complements that of the vinyl chloride resin so that the dispersion and coatings properties of the blends are better than those of either component alone.